The interplay between cognitive ability, alcohol consumption, and health characteristics.

BACKGROUND: Higher cognitive ability is associated with favourable health characteristics. The relation between ability and alcohol consumption, and their interplay with other health characteristics, is unclear. We aimed to assess the relationship between cognitive ability and alcohol consumption and to assess whether alcohol consumption relates differently to health characteristics across strata of ability. METHODS: For 63 120 Norwegian males, data on cognitive ability in early adulthood were linked to midlife data on alcohol consumption frequency (times per month, 0-30) and other health characteristics, including cardiovascular risk factors and mental distress. Relations were assessed using linear regression and reported as unstandardised beta coefficients [95% confidence interval (CI)]. RESULTS: The mean ± s.d. frequency of total alcohol consumption in the sample was 4.0 ± 3.8 times per month. In the low, medium, and high group of ability, the frequencies were 3.0 ± 3.3, 3.7 ± 3.5, and 4.7 ± 4.1, respectively. In the full sample, alcohol consumption was associated with physical activity, heart rate, fat mass, smoking, and mental distress. Most notably, each additional day of consumption was associated with a 0.54% (0.44-0.64) and 0.14% (0.09-0.18) increase in the probability of current smoking and mental distress, respectively. In each strata of ability (low, medium, high), estimates were 0.87% (0.57-1.17), 0.48% (0.31-0.66) and 0.49% (0.36-0.62) for current smoking, and 0.44% (0.28-0.60), 0.10% (0.02-0.18), and 0.09% (0.03-0.15) for mental distress, respectively. CONCLUSIONS: Participants with low cognitive ability drink less frequently, but in this group, more frequent alcohol consumption is more strongly associated with adverse health characteristics.

A longitudinal, population-based twin study of avoidant and obsessive-compulsive personality disorder traits from early to middle adulthood.

BACKGROUND: The phenotypic stability of avoidant personality disorder (AVPD) and obsessive-compulsive personality disorder (OCPD) has previously been found to be moderate. However, little is known about the longitudinal structure of genetic and environmental factors for these disorders separately and jointly, and to what extent genetic and environmental factors contribute to their stability. METHOD: AVPD and OCPD criteria were assessed using the Structured Interview for DSM-IV Personality in 2793 young adult twins (1385 pairs, 23 singletons) from the Norwegian Institute of Public Health Twin Panel at wave 1 and 2282 (986 pairs, 310 singletons) of these on average 10 years later at wave 2. Longitudinal biometric models were fitted to AVPD and OCPD traits. RESULTS: For twins who participated at both time-points, the number of endorsed sub-threshold criteria for both personality disorders (PDs) decreased 31% from wave 1 to wave 2. Phenotypic correlations between waves were 0.54 and 0.37 for AVPD and OCPD, respectively. The heritability estimates of the stable PD liabilities were 0.67 for AVPD and 0.53 for OCPD. The genetic correlations were 1.00 for AVPD and 0.72 for OCPD, while the unique environmental influences correlated 0.26 and 0.23, respectively. The correlation between the stable AVPD and OCPD liabilities was 0.39 of which 63% was attributable to genetic influences. Shared environmental factors did not significantly contribute to PD variance at either waves 1 or 2. CONCLUSION: Phenotypic stability was moderate for AVPD and OCPD traits, and genetic factors contributed more than unique environmental factors to the stability both within and across phenotypes.

A longitudinal twin study of cluster A personality disorders.

BACKGROUND: While cluster A personality disorders (PDs) have been shown to be moderately heritable, we know little about the temporal stability of these genetic risk factors. METHOD: Paranoid PD (PPD) and schizotypal PD (STPD) were assessed using the Structured Interview for DSM-IV Personality in 2793 young adult twins from the Norwegian Institute of Public Health Twin Panel at wave 1 and 2282 twins on average 10 years later at wave 2. Using the program Mx, we fitted a longitudinal latent factor model using the number of endorsed criteria for PPD and STPD. RESULTS: The stability over time of the criteria counts for PPD and STPD, estimated as polychoric correlations, were +0.34 and +0.40, respectively. The best-fit longitudinal model included only additive genetic and individual-specific environmental factors with parameter estimates constrained to equality across the two waves. The cross-wave genetic and individual-specific environmental correlations for a latent cluster A factor were estimated to equal +1.00 and +0.13, respectively. The cross-time correlations for genetic and environmental effects specific to the individual PDs were estimated at +1.00 and +0.16-0.20, respectively. We found that 68% and 71% of the temporal stability of PPD and STPD derived, respectively, from the effect of genetic factors. CONCLUSION: Shared genetic risk factors for two of the cluster A PDs are highly stable in adults over a 10-year period while environmental risk factors are relatively transient. Over two-thirds of the long-term stability of the common cluster A PD liability can be attributed to genetic influences.

A longitudinal twin study of borderline and antisocial personality disorder traits in early to middle adulthood.

BACKGROUND: Antisocial personality disorder (ASPD) and borderline personality disorder (BPD) share genetic and environmental risk factors. Little is known about the temporal stability of these etiological factors in adulthood. METHOD: DSM-IV criteria for ASPD and BPD were assessed using structured interviews in 2282 Norwegian twins in early adulthood and again approximately 10 years later. Longitudinal biometric models were used to analyze the number of endorsed criteria. RESULTS: The mean criterion count for ASPD and BPD decreased 40% and 28%, respectively, from early to middle adulthood. Rank-order stability was 0.58 for ASPD and 0.45 for BPD. The best-fitting longitudinal twin model included only genetic and individual-specific environmental factors. Genetic effects, both those shared by ASPD and BPD, and those specific to each disorder remained completely stable. The unique environmental effects, however, changed substantially, with a correlation across time of 0.19 for the shared effects, and 0.39 and 0.15, respectively, for those specific to ASPD and BPD. Genetic effects accounted for 71% and 72% of the stability over time for ASPD and BPD, respectively. The genetic and environmental correlations between ASPD and BPD were 0.73, and 0.43, respectively, at both time points. CONCLUSION: ASPD and BPD traits were moderately stable from early to middle adulthood, mostly due to genetic risk factors which did not change over the 10-year assessment period. Environmental risk factors were mostly transient, and appear to be the main source of phenotypic change. Genetic liability factors were, to a large extent, shared by ASPD and BPD.

Hearing status among Norwegian train drivers and train conductors.

BACKGROUND: There is a general perception that train drivers and conductors may be at increased risk of developing noise-induced hearing loss. AIMS: To study job-related hearing loss among train drivers and train conductors. METHODS: Audiograms from train drivers and train conductors were obtained from the medical records of the occupational health service of the major Norwegian railway company. The results were compared with audiograms from an internal control group of railway workers and an external reference group of people not occupationally exposed to noise. The monaural hearing threshold level at 4kHz, the mean binaural value at 3, 4 and 6kHz and the prevalence of audiometric notches (≥25 dB at 4kHz) were used for comparison. RESULTS: Audiograms were available for 1567 drivers, 1565 conductors, 4029 railway worker controls and 15 012 people not occupationally exposed to noise. No difference in hearing level or prevalence of audiometric notches was found between study groups after adjusting for age and gender. CONCLUSIONS: Norwegian train drivers and conductors have normal hearing threshold levels comparable with those in non-exposed groups.

Genetic and environmental contributions to the relationship between education and anxiety disorders - a twin study.

OBJECTIVE: To examine the negative statistical relationship between educational level and risk of anxiety disorders, and to estimate to what extent this relationship may be explained by genes or environmental factors influencing both phenotypes. METHOD: Registry data on educational level for 3339 young adult Norwegian twin pairs and diagnostic data on anxiety disorders for 1385 of these pairs were analysed, specifying structural equations models using MX software. RESULTS: In the best-fitting model, genes accounted for 59% of the variance in education. 18% of the variance was due to environmental factors shared by co-twins, and the remaining 23% due to non-shared environment; 46% of the variance in liability to anxiety disorders was genetic, the remaining variance was due to non-shared environment. A phenotypic polychoric correlation of -0.30 between educational level and 'any anxiety disorder' was estimated to be primarily (83% in the best-fitting model) caused by genes common to the two traits. CONCLUSION: The relationship between low education and risk of anxiety disorders appears to be primarily determined by genetic effect common to educational level and anxiety disorders.

The structure of genetic and environmental risk factors for phobias in women.

BACKGROUND: To explore the genetic and environmental factors underlying the co-occurrence of lifetime diagnoses of DSM-IV phobia. METHOD: Female twins (n=1430) from the population-based Norwegian Institute of Public Health Twin Panel were assessed at personal interview for DSM-IV lifetime specific phobia, social phobia and agoraphobia. Comorbidity between the phobias were assessed by odds ratios (ORs) and polychoric correlations and multivariate twin models were fitted in Mx. RESULTS: Phenotypic correlations of lifetime phobia diagnoses ranged from 0.55 (agoraphobia and social phobia, OR 10.95) to 0.06 (animal phobia and social phobia, OR 1.21). In the best fitting twin model, which did not include shared environmental factors, heritability estimates for the phobias ranged from 0.43 to 0.63. Comorbidity between the phobias was accounted for by two common liability factors. The first loaded principally on animal phobia and did not influence the complex phobias (agoraphobia and social phobia). The second liability factor strongly influenced the complex phobias, but also loaded weak to moderate on all the other phobias. Blood phobia was mainly influenced by a specific genetic factor, which accounted for 51% of the total and 81% of the genetic variance. CONCLUSIONS: Phobias are highly co-morbid and heritable. Our results suggest that the co-morbidity between phobias is best explained by two distinct liability factors rather than a single factor, as has been assumed in most previous multivariate twin analyses. One of these factors was specific to the simple phobias, while the other was more general. Blood phobia was mainly influenced by disorder specific genetic factors.

Family matters: happiness in nuclear families and twins.

Biometric studies have shown that happiness is strongly affected by genes. The findings are mainly based on twin data, however, and the full validity of the results has been debated. To overcome some limitations in classical twin research, we examined aetiological sources of subjective well-being (SWB), using two independent population-based samples, one including nuclear families (N = 54,540) and one including twins (N = 6,620). Biometric modelling using R was conducted to test for a data structure implying either non-additive genetic effects or higher environmental co-twin correlation in MZ than DZ pairs (violation of the EEA). We also estimated non-random mating, cultural transmission and shared environments specific for regular siblings and twins. Two sets of nested models were fitted and compared. The best explanatory model shows that family matters for happiness predominantly due to quantitative sex-specific genetic effects, a moderate spousal correlation and a shared twin environment. Upper limits for broad-sense heritability were estimated to be 0.33 (females) and 0.36 (males). Our study constitutes the most elaborate biometric study of SWB to date and illustrates the utility of including responses from multiple types of relatives in quantitative genetic analyses.

Does a higher educational level protect against anxiety and depression? The HUNT study.

The relationship of education to the experience of anxiety and depression throughout adult life is unclear. Our knowledge of this relationship is limited and inconclusive. The aim of this study was to examine (1) whether higher educational level protects against anxiety and/or depression, (2) whether this protection accumulates or attenuates with age or time, and (3) whether such a relationship appears to be mediated by other variables. In a sample from the Nord-Trøndelag Health Study 1995--1997 (HUNT 2) (N=50,918) of adults, the cross-sectional associations between educational level and symptom levels of anxiety and depression were examined, stratified by age. The long-term effects of educational level on anxiety/depression were studied in a cohort followed up from HUNT 1 (1984--1986) to HUNT 2 (N=33,774). Low educational levels were significantly associated with both anxiety and depression. The coefficients decreased with increasing age, except for the age group 65-74 years. In the longitudinal analysis, however, the protective effect of education accumulated somewhat with time. The discrepancy between these two analyses may be due to a cohort effect in the cross-sectional analysis. Among the mediators, somatic health exerted the strongest influence, followed by health behaviors and socio-demographic factors. Higher educational level seems to have a protective effect against anxiety and depression, which accumulates throughout life.

The relationship between anxiety disorders and dimensional representations of DSM-IV personality disorders: A co-twin control study.

BACKGROUND: There is substantial comorbidity between personality disorders (PDs) and anxiety disorders (ADs). Sharing of familial risk factors possibly explains the co-occurrence, but direct causal relationships between the disorders may also exist. METHODS: 2801 persons from 1391 twin pairs from the Norwegian Institute of Public Health Twin Panel were assessed for all DSM-IV PDs and ADs. Bivariate Poisson-regression analyses were performed to assess whether PDs predicted ADs at three different levels: All PDs combined, PDs combined within DSM-IV-clusters and each individual PD separately. Next, bivariate co-twin control analyses were executed within monozygotic (MZ) and dizygotic (DZ) twin pairs. A similar analytic strategy was employed in multivariate models including PDs as independent variables. RESULTS: PDs predicted ADs at all levels of analysis in bivariate regression models. Bivariate co-twin control analyses demonstrated an increased risk of ADs in all PDs combined, all PD-clusters and in schizotypal, paranoid, borderline, antisocial, avoidant and dependent PD. In the multivariate regression model, all PD-clusters and schizotypal, borderline, avoidant and obsessive-compulsive PD predicted ADs. Only borderline and avoidant PD predicted ADs in the multivariate co-twin control analysis. LIMITATIONS: Over-adjustment may explain the results from the multivariate analyses. The cross-sectional study design hampers causal inference. CONCLUSIONS: Comorbidity between ADs and PDs can be largely accounted for by shared familial risk factors. However, the results are also consistent with a direct causal relationship partly explaining the co-occurrence. Our results indicate specific environmental factors for comorbidity of ADs and borderline and avoidant PDs that are not shared with other PDs.

The Nord-Trøndelag Norway Audiometric Survey 1996-98: unscreened thresholds and prevalence of hearing impairment for adults > 20 years.

As supplement to a general health screening examination (HUNT-II), we conducted a puretone audiometry study in 1996-98 on adults (>20 years) in 17 of 23 municipalities in Nord-Trøndelag, Norway, including questionnaires on occupational and leisure noise exposure, medical history, and symptoms of hearing impairment. The study aims to contribute to updated normative hearing thresholds for age and gender, while evaluating the effects of noise exposure, medical history, and familial or genetic influences on hearing. This paper presents the unscreened hearing threshold data and prevalence of hearing impairment for different age groups and by gender. Valid audiometric data were collected from 62% (n=50,723) of 82,141 unscreened invited subjects (age-range 20-101 years, mean=50.2 years, SD=17.0 years). Two ambulant audiometric teams each conducted 5 parallel self-administered, pure-tone hearing threshold examinations with the standard test frequencies 0.25-0.5-1-2-3-4-6-8 kHz (manual procedure when needed). Tracking audiometers were used in dismountable booths with in-booth noise levels well within ISO criteria, except being at the criterion around 200 Hz. The data were electronically transferred to a personal computer. Test-retest correlations for 99 randomly drawn subjects examined twice were high. The mean thresholds recorded were some dB elevated from "audiometric zero" even for age group 20-24 years. As also found in other studies, this might indicate too restrictive audiometric reference thresholds. Males had slightly better hearing < or =0.5 kHz for all age groups. Mean thresholds were poorer in males > or = 30 years from > or =2 kHz, with maximal gender differences of approximately 20 dB at 3-4 kHz for subjects aged 55-74 years. Weighted prevalence data averaged over 0.5-1-2-4 kHz showed hearing impairment >25 dB hearing threshold level of 18.8% (better ear) and 27.2% (worse ear) for the total population--for males 22.2% and 32.0%, for females 15.9% and 23.0%, respectively. Mean hearing loss > or =10 dB at 6 kHz registered for both genders even in age groups 20-24 years may be partly due to calibration artefacts, but might possibly also reflect noise-related socio-acusis.

Age-specific genetic effects for blood pressure.

Correlations between relatives were determined for systolic and diastolic blood pressure. The correlations decrease as age differences between relatives increase in a Norwegian sample with 43,751 parent-offspring pairs, 19,140 pairs of siblings, and 169 pairs of twins. A simple biometric model specifying only age-specific genetic additive effects and environmental effects fitted well to correlations between cotwins, pairs of siblings, and parent-offspring dyads in subsets of relatives grouped by age differences. None of the environmental effects appeared to be due to environmental factors that are shared by family members. Models that excluded a parameter for the age-specific genetic influence did not fit the data. The results may partly explain what seems to be a discrepancy between relatively low parent-offspring correlations from previous nuclear family studies and high correlations from twin studies, especially in identical twins.

Manual work as predictor for disability pensioning with osteoarthritis among the employed in Norway 1971-1990.

BACKGROUND: Manual work is reported to be a risk factor for becoming a disability pensioner due to osteoarthritis. This association may be due to covariation with other variables. We wanted to assess if manual work remained a risk factor after adjusting for number of hours worked, income, level of education, gender and marital status, and if the risk associated with manual work was equal in the 1970s and the 1980s. METHODS: In a prospective study, data on all new disability pensioners with osteoarthritis in Norway during the two follow-up periods, 1971-1980 and 1981-1990, were analysed by logistic regression. The study include data on all subjects living in Norway and registered as 50-56 years old and employed either in the census collected in 1970 or in the census of 1980. RESULTS: Manual workers have nearly twice the probability of becoming a disability pensioner with osteoarthritis compared to professionals after adjusting for part-time work, income, level of education, marital status and gender. Adjusted for other risk factors, the probability of becoming a disability pensioner with osteoarthritis was three times higher in the 1980s compared to the 1970s. CONCLUSION: The relatively strong association between manual work and disability pensioning with osteoarthritis suggests difficulties in adjusting manual work patterns for a person with osteoarthritis, which may have increased during the study period as implied by the separate effect of the 1980s.

Is there a consequence for fetal growth of having an unlike-sexed cohabitant in utero?

BACKGROUND: There is evidence to suggest a masculinizing effect on female intrauterine development in unlike-sexed twins. The purpose of the present report was to examine the possible effects of male presence on fetal growth in females by comparing mean birthweights in members from dizygotic unlike-sexed (DZU) pairs with those from dizygotic like-sexed (DZL) pairs. METHODS: The sample consisted of 1087 DZU and 1089 DZL twin pairs from the New Norwegian Twin Panel, which was established by identifying all twin births from 1967 to 1974 through the population-based Medical Birth Registry. RESULTS: The mean birthweight of females from DZU pairs was 2684+/-15 g (+/-SEM), as opposed to 2647+/-19 g in females from DZL pairs (P = 0.06). For males, the mean birthweight was 2812+/-16 g in DZU pairs and 2805+/-20 g in DZL pairs (P= 0.78). CONCLUSION: We found a tendency for the birthweight in females to be influenced by the presence of a male co-twin. This observation may have a biological significance and should lead to a close follow-up of DZU and DZL females with respect to hormone-sensitive disorders and reproductive ability.

Transmission of symptoms of anxiety and depression in nuclear families.

Anxiety and depression symptom scores from the SCL-90 questionnaire were observed in a large sample of nuclear families, and the effects of genes or family environment were estimated. Assuming no environmental transmission from parents to offspring, heritability was estimated at 0.43 for both anxiety and depression scores, and common sibling environment or reciprocal sibling influence explained 19% of the variances for both scores. Most of the (genetic or environmental) family effect seemed to be common for the two scores. There was no evidence of common determinants in the family for the symptom scores and alcohol consumption, not even of any substantial relationship within persons between symptoms and consumption. The spouse correlations were 0.25 for anxiety and 0.38 for depression. Similar values for the correlations between anxiety in one person and depression in his/her spouse implies a mate selection based on a single latent variable to which the two observed traits contribute.

Low genetic effect and age-specific family effect for symptoms of anxiety and depression in nuclear families, halfsibs and twins.

Questionnaire information on symptoms of anxiety/depression was obtained from 61,286 persons, most of whom could be grouped in families as spouses, parents, offspring, siblings, halfsibs and twins. The correlations between mental health in relatives, varying somewhat by sex, were: 0.27 for 18,768 pairs of spouses, 0.12 to 0.17 for 30,888 pairs of parents and offspring, 0.11 to 0.18 for 13,134 pairs of siblings, 0.27 to 0.35 for 57 pairs of MS twins, -0.06 to 0.10 for 60 pairs of DZ twins, 0.30 for 88 pairs of halfsibs reared together, and 0.16 for 40 pairs of halfsibs reared apart. The heritability was estimated to be between 0 and 0.20. There was a significant effect of environmental transmission from mother, but not from father, to offspring. The effect of environmental factors shared by sisters was substantial, the corresponding effect was lower in brothers, and zero in siblings of opposite sex. The correlations decreased with increasing age differences between relatives, implying effects of age-specific genes or environmental factors in the family.

Symptoms of anxiety and depression among mothers of pre-school children: effect of chronic strain related to children and child care-taking.

BACKGROUND: Factors that affect maternal mental health were studied when the children were 30 and 50 months old, and changes in the importance of these factors over time were analyzed. A specific aim was to elucidate the role of chronic strain related to children and child care-taking. This study follows up previous work on the influence of social class, strain and social support on maternal mental distress when the children were 18 months old. METHODS: The sample is population based, and 1,081 parents were invited to fill out questionnaires. Maternal mental distress was measured by the Hopkins Symptom Checklist (SCL-25). Multiple regression analyses were conducted at each time point and chi-square tests were used to analyze the changes between the estimated regression coefficients over time. RESULTS: Chronic strain related to children and child care-taking consistently predicted maternal mental distress. Among the specific child related strains, problems with child care-taking were significantly associated with maternal symptom levels at all time points. The importance of two specific child problem behaviors (activity level and the child being a worrier) on maternal mental health changed over time. LIMITATIONS: Conclusions about causality can not be drawn based on cross-sectional analyses. The self-report measures used here may be biased by the current mood state. CONCLUSIONS: Problems with child care arrangements and combining work and child care-taking are predictive of maternal mental health when the children are 18, 30 and 50 months old. The risk and protective factors found here may have implications for prevention and intervention.

Socioeconomic factors and disability retirement from back pain: a 1983-1993 population-based prospective study in Norway.

STUDY DESIGN: A prospective observational study with an 11-year follow-up period was performed. OBJECTIVE: To investigate the influence of education and socioeconomic position on the incidence of permanent disability retirement from back pain. SUMMARY OF BACKGROUND DATA: Early retirement because of back pain is the extreme end point of a disabling process that is a great burden to the individual and costly for the society. Groups of employees at particular risk for permanent back pain disability need to be identified. METHODS: All employed men and women in Norway between the ages of 20 and 53 years in 1980 were included (n = 1,333,556). Outcome measures were disability retirement from inflammatory back pain (ICD-9 code 720) and noninflammatory back pain (ICD-9 codes 721 to 724). RESULTS: The 11-year cumulative incidence was 0.15% (n = 1990) for disability retirement from inflammatory back pain and 1. 64% (n = 21,829) for noninflammatory back pain and was somewhat higher in women than in men. Each year of formal education was independently associated with decreased risk for disability retirement from noninflammatory back pain (odds ratio [OR] = 0.78; 95% confidence interval [CI] = 0.77-0.79) and from inflammatory back pain (OR = 0.83; 95% CI = 0.81-0.86). Whereas disability from inflammatory back pain was moderately associated with socioeconomic status, there was a consistent upward trend in the association between disability retirement from noninflammatory back pain and lower socioeconomic position. The OR for unskilled workers was 3.1 (95% CI = 2.6-3.7) for men and 2.1 (95% CI = 1.7-2.5) for women, as compared with that of higher professionals. Stepwise analyses suggest that the effect of education is not mediated by socioeconomic status. CONCLUSIONS: The consistent upward trend in the relation of disability retirement to lower levels of education and socioeconomic position, even for inflammatory back pain, shows that factors related to the occupational and social environment play an important role in the disabling process. The stepwise, monotonic relation between socioeconomic position and disability retirement from back pain, even at the higher end of the socioeconomic scale, suggests that the relation between social class and back pain disability cannot be explained solely in terms of manual versus nonmanual jobs.

Depression during pregnancy and after delivery: a repeated measurement study.

The aim of this study was to examine the risk of depression in the postpartum period (first four months after delivery) as compared to the remaining postnatal year and the pregnancy period. All postpartum women from two municipalities in Norway were included in a questionnaire study of mental health (n = 416). Over 50% of the women (n = 259) answered an identical questionnaire at an additional time either before or after the postpartum period. The level of depression was measured by the Edinburgh Postnatal Depression Scale (EPDS) and the Hopkins Symptom Check List-25 items (SCL-25). The point prevalence of depression (EPDS> or =10) in the first four months postpartum did not differ significantly as compared to other time periods during pregnancy and the postnatal year. This finding remained also after controlling for other risk factors of depression; high score on the life event scale, prior depression and poor partner relationship. There was a non-significant trend of lower prevalence of depression during early pregnancy and after the first eight postnatal months. In conclusion, our findings suggest that the first four months postpartum were not distinguished by higher depression prevalence as compared to other time periods during pregnancy and the first postnatal year.

Distribution and heritability of recurrent ear infections.

The distribution of recurrent ear infections was obtained from a population-based sample of 2,750 pairs of Norwegian twins born between 1967 and 1974. The lifetime prevalence of self-reported recurrent ear infections was 8.9%, with a significant predominance of female cases. The mean age of onset was 4.2 years, with a gradual decrease in occurrence from 2 to 7 years of age. Among monozygotic pairs, the rate of tetrachoric correlation between co-twins was almost identical in males (0.73, SE 0.08) and females (0.74, SE 0.06), but among the dizygotic pairs the correlation was clearly higher in males (0.53, SE 0.12) than in females (0.20, SE 0.12). The value in the unlike-sexed dizygotic twins (0.25, SE 0.05) was intermediate to that of the like-sexed male and female dizygotic pairs. The relative contribution of genes and environment to variability in the predisposition to develop otitis media was estimated by means of structural equation modeling. Variation in liability to ear infections was mainly explained by additive genetic and dominance factors in females, for whom heritability was estimated at 74%. The remaining 26% of the variation in liability was explained by individual environmental factors. In males, 45% of the variation could be accounted for by genetic factors, 29% by common familial environment, and the remaining 26% by individual environmental effects.